1. Field of the Invention
This invention relates to a key switch capable of use in a keyboard, specifically to a structure of the key switch.
2. Description of the Related Art
In the related art, many key switches are disposed on a keyboard, which is used in computer system, subject to specific rules. Each of the key switches includes a key top on which a letter is printed, an elastic member made of a rubber material, a membrane contact sheet and a base plate. The elastic member includes a bell-shaped spring leg part, an upper part and a contact depression part. One end of the bell-shaped spring leg part is opened, and the other end is connected to the upper part. The contact depression part is formed at a center in the internal surface of the bell-shaped spring leg part. The key top is formed on the upper part of the elastic member. The elastic member is disposed on the membrane contact sheet in contacting its open end to the membrane contact sheet.
The membrane contact sheet includes a pair of flexible sheets (upper and lower flexible sheets), which face each other, and an insulating resin sheet, which is sandwiched between the flexible sheets. The lower flexible sheet is formed on the base plate and the upper flexible sheet contacts the open end of the elastic member. The insulating resin sheet includes an opening, which is concentric with the open end of the elastic member. By forming the opening, a room is defined between the flexible sheets. Each of the flexible sheets has a contacting pad, which is located in the opening, and the contacting pads formed on the respective flexible sheets face each other. Each of the contacting pads is connected to a conductive wire, which is formed on each flexible sheet. The conductive wire is formed of conductive ink made of silver, carbon and other materials, and is printed on the flexible sheet. The conductive wire has a width of about 0.5 mm and a height of about 10 xcexcm.
According to this structure, when the key top, in an original position, is pushed down, the bell-shaped spring leg part is deformed so that the key top is moved down so that the contact depression part contacts the upper flexible sheet. When the key top is pushed further down, the upper flexible sheet is deformed toward the lower flexible sheet. Since the contact depression part contacts the upper flexible sheet in an area where the contacting pad is formed, the contact pads formed on the respective flexible sheets contact each other when the key top is pushed down further. In this state, the key switch is closed. When the key top is released, the key top returns to the original position by the memory of the elastic member, and the upper flexible sheet returns to its original shape by its memory. As a result, the contacting pads are separated so that the key switch is opened.
Two ways to satisfy a requirement for downsizing a keyboard having the structure described above include: (1) narrowing an interval between the key switches, (2) reducing the size of the key switch itself. However, narrowing the interval has the problem that there is a definite limit to narrowing an interval between the key switches. This is because it is necessary to secure a space for printing the conductive wire.
Reducing the size of the key switch has the following problem. The diameters of the open end of the bell-shaped spring leg part and the opening of the insulating resin sheet must be reduced. Here, the diameter of the open end of the bell-shaped spring leg part is often larger than that of the opening of the insulating resin sheet. Thus, the bell-shaped spring leg part is supported by the insulating resin sheet.
Further, in a case that the diameter of the opening of the insulating resin sheet is about 3.5 mm, it is required to push the key top with a force in the range between 7 gf and 9 gf in order to make the contact pads contact. In another case, where the diameter of the opening of the insulating resin sheet is about 3.0 mm, it is required to push the key top with a force in the range between 13 gf and 15 gf in order to make the contact pads contact. Accordingly, the smaller the diameter of the opening is, the larger the required power is.
On the other hand, when the internal diameter of the bell-shaped spring leg part is reduced without reducing the length of the diameter of the opening, in other words, when the external diameter of the bell-shaped spring leg part equals or is less than the diameter of the opening of the insulating resin sheet, the force for pushing the key top down is applied to the upper flexible sheet on the opening via the bell-shaped spring leg part. Thus, the upper flexible sheet is deformed easily by less force so that the contact pads sometimes unintentionally contact each other.
Accordingly, in the related arts, to avoid these problems, the open end of the bell-shaped spring leg part is placed above the insulating resin sheet so that a relatively large elastic member is formed. This makes it difficult to downsize the key switches.
Further, it is preferable that a center of the contact depression part is brought into line with a center of each contact pad. If these are misaligned, the force for pushing the key top down to contact the contact pads varies. For example, if the diameter of the opening is about 3.5 mm, when the center of the contact depression part is misaligned with the center of each contact pad by 0.5 mm, it is required to push the key top with a force in the range between 8 gf and 12 gf in order to make the contact pads contact. In another case where the diameter of the opening is about 3.0 mm, when the center of the contact depression part is misaligned with the center of each contact pad by 0.5 mm, it is required to push the key top with a force in the range between 14 gf and 18 gf in order to make the contact pads contact.
As described above, compared to a key switch having no misalignment between the center of the contact depression part and the center of each contact pad, when the center of the contact depression part is misaligned with the center of each contact pad because of a misprint of the insulating resin sheet or misassembly of the elastic member, a larger force is required to push the key top down. Thus, when the key top is pushed with the intent to push the key switch having no misalignment, the contact pads may not contact each other because of the lack of the force.
It is therefore an object of the invention to resolve the above-described problem in a key switch and provide a downsized key switch.
It is another object to provide a key switch, which is operated by the constant force.
It is yet another object to provide a key switch, which is operated by the stable force.
It is still another object to provide a key switch, which is lax in accuracy of its assembling.
The object is achieved by a key switch including a key top, an elastic member, a first flexible sheet, a moveable contact pad, a second flexible sheet, a fixed contact pad, a insulating resin sheet. The elastic member includes a top part disposed under the key top, a bell-shaped spring leg having a circular open end, and a contact depression part located inside the bell-shaped spring leg. The first flexible sheet includes a top surface and a back surface, and the elastic member is placed on the top surface. The moveable contact pad is formed on the back surface of the first flexible sheet, and the moveable contact pad is located at the center of the open end of the bell-shaped spring leg. The fixed contact pad is formed on the second flexible sheet, and faces the moveable contact pad. The insulating resin sheet is sandwiched between the first and second flexible sheets, and the insulating resin sheet includes an opening, which has a shape different from that of the circular open end of the bell-shaped spring leg. The shortest distance between the center of the opening and the periphery of the opening is shorter than a half of the inside diameter of the circular open end. The longest distance between the center of the opening and the periphery of the opening is longer than the half of the inside diameter of the circular open end.